Mr Sang on the Construction of Circular Towers. 247 



portional to the height of the hydrostatic column, and to the 

 compressibility of the substance. 



When the cement is of the same nature with the grains, 

 both expand alike ; and hence the mutual action of the parts 

 is not affected ; but when the cement is of a different nature 

 from the grains, the removal of the hydrostatic pressure will 

 occasion one of two classes of effects. If the cement be 

 more compressible than the grains, it will expand more, and 

 separate them somewhat from each other ; the aggregate will 

 possess all the cohesive power of the cement. If, on the other 

 hand, the cement be less compressible than the grains, the 

 superior expansiveness of these will, on the removal of the 

 hydrostatic pressure, distend the cement, and the cohesiveness 

 of the mass will be diminished. The sandstone having been 

 dried, let the weight of the superior strata be removed ; the 

 cement was not subjected to any strain on account of that 

 weight, it will not expand ; but the grains which supported 

 the whole of this weight will expand vertically, and will thus 

 distend the cement, and diminish its cohesion. 



In all cases, then, the cohesion across the strata, of a stra- 

 tified rock, must be diminished by the simple removal of it 

 from its site, while, in some cases, the cohesion in the direc- 

 tion of the strata is also diminished by the mere drying of 

 the rocks. 



The accuracy of these inferences is well established by 

 many familiar examples, among which the spontaneous crum- 

 bling of various shales is conspicuous. It is to be regretted 

 that we have as yet no accurate experiments on the compressi- 

 bilities of mineral substances, otherwise the comparative cohe- 

 sive strengths of various species of stratified rock might be to 

 some extent inferred. 



It thus follows, generally, that stratified rocks are capable 

 of but slender resistance to distension ; besides, having but a 

 limited range of stretching, they can resist but a very slight 

 blow, — a circumstance which our stone-masons ai-e in daily 

 habit of rendering available, and of which the more conspi- 

 cuous phases are illustrated in the freestone and the slate 

 quarries. 



The formation of igneous or massive rocks is less intelli- 



